"The OFT has launched an investigation into whether children are being unfairly pressured or encouraged to pay for additional content in 'free' web and app-based games, including upgraded membership or virtual currency such as coins, gems or fruit. Typically, players can access only portions of these games for free, with new levels or features, such as faster game play, costing money." Instances of this may be illegal, especially when it targets children. As for me - I just find it incredibly annoying.

Always require a password for every purchase. People have "1 Click Shopping" on devices that they then hand to their toddlers. It's replacing commercials that have your kids saying, "Mom! Dad! I want that!" with them just purchasing it with their parents' credit card without the parents knowing.